1. Field of the Invention
The present invention relates generally to optical detection systems for use in multicapillary analytical instrumentation. More particularly, the present invention involves apparatus and methods for automatically aligning an electromagnetic beam with the centers of each of a plurality of capillaries which collectively form a capillary array.
2. Description of Relevant Art
Electrophoretic separation techniques have been utilized for decades to separate molecules according to differences in the effective charge of the molecules, and/or according to differences in the molecular size of the molecules. Up until recently electrophoretic separations were conducted in gel slabs or open gel beds which were typically fabricated of polyacrylamide gel material. More recently capillary electrophoresis techniques combined with photometric detection methods have allowed the automation and rapid quantitative analysis of molecules. High resolution separations of molecules having different effective charges have been achieved by applying electrophoretic principles to buffer filled or gel filled narrow capillary tubes.
Typically, capillary columns used in capillary electrophoresis are fabricated of lengths of silica tubing having diameters on the order of 25 .mu.m to 200 .mu.m and lengths from about 10 to 200 cm. The buffer and gel separation mediums are pumped directly into the column interiors, analytical samples are moved into one end of the column, and an electric field is applied to the column. Charged species within the column migrate under the influence of the electric field, at a rate which depends upon their electrophoretic mobility. Capillary electrophoresis is used to separate numerous types of molecules including peptides, proteins, and oligonucleotides, nucleic acids and other charged molecular species. The field of electrophoretic separation technology is continually expanding with respect to the types and sizes of molecules which can be separated and detected using capillary electrophoresis procedures.
The advantages associated with capillary electrophoresis are numerous. Quantitative information can be achieved with very small sample sizes, and the amount of gel or buffer consumed is minuscule. Furthermore, the time required for the separations is sharply reduced when compared with slab gel techniques, and the technique lends itself to automation and electronic data storage and data manipulation.
Recently, coupling laser induced fluorescence (LIF) detectors with capillary electrophoresis instrumentation (CE-LIF) has improved the detection sensitivity associated with capillary electrophoresis by orders of magnitude. This has resulted in the ability to detect subpicomolar quantities of fluorescently tagged molecules within a capillary. Because capillary electrophoresis, and in particularly CE-LIF technology, have provided for the rapid automated separation and detection of minute quantities of material, it is particularly attractive as a separation and analysis technique in applications in which only microliters of sample volume containing nanomolar concentrations of analyte are available. One drawback associated with traditional capillary electrophoresis systems is its overall sample throughput. Traditional slab gel electrophoresis systems typically require longer run times, but each slab is capable of analyzing a large number of samples, making the number of separations within a given time period greater. For example, a typical capillary electrophoresis separation and analysis may require only 15 minutes for a single sample, or four separations per hour. In contrast, a slab gel separation may accommodate ten samples and require a one-hour run time, or ten separations per hour.
In addressing the overall throughput problem, several multicapillary capillary electrophoresis systems have been suggested. One such system is described in U.S. Pat. Nos. 5,091,652 and 5,274,240. Another system, described in copending U.S. patent Ser. No. 08/429,406, utilizes an array of capillary columns arranged in a ribbon like configuration and a single detector for detecting analyte in all the columns. In this galvoscanner based detection system a galvometric scanner mirror is caused to move in a step-wise fashion in order to direct focused electromagnetic radiation sequentially to each capillary in the array. In order to achieve the highest possible detector sensitivity and most efficient duty cycle, it is desirable that the focused beam illuminate or interrogate each capillary at its center. This assures that the time the illuminating beam is targeted on-center is maximized and the time the beam is off-center is minimized.
Because the capillaries of multicapillary electrophoresis systems are frequently replaced by the user and may be physically disrupted between runs due to small temperature changes or pressure fluctuations caused by replacing gel and/or buffers, the ability to align accurately a source beam with respect to the center of each capillary improves performance of the system. In fact, the ability to rapidly and automatically align a focused source beam position with respect to the center of each capillary prior to each electrophoretic run in a pre-scan alignment step is highly desirable.